Torsional vibration isolation mechanisms have long been used to reduce the adverse effects of fluctuating torques or torsional vibrations in vehicle drivelines. A need to improve vehicle efficiency has necessitated developments that have dramatically increased transmission gear rattle noise. For example, engine efficiency has been improved by reducing the number of engine cylinders; unfortunately, this has increased torque fluctuations which cause gear rattle. Further, transmissions have been made more susceptible to gear rattle by adding speed ratios, by reducing parasitic friction, and by generally increasing transmission resonance to frequencies which occur at engine speeds in the vehicle's normal driving range.
The prior art, two mass flywheel assembly shown in FIG. 2 has been particularly effective in reducing gear rattle in driveline systems employing a master clutch which selectively connects an engine flywheel to a transmission input shaft. The assembly includes primary and secondary flywheel masses and a torsional isolation mechanism disposed between the masses. The isolation mechanism is relocated from the clutch plate of the master clutch, thereby simplifying the clutch plate and reducing its inertia which forms a continuous part of the transmission input shaft inertia. The reduced inertia facilitates quicker and easier synchronization of transmission ratio gears when the master clutch is disengaged. The primary flywheel mass is fixed to the engine shaft and the secondary flywheel mass is connected to the transmission input shaft only when the master clutch is engaged. When the secondary mass is connected to the input shaft, its added inertia lowers transmission resonance to frequencies less than engine speeds in the vehicle's normal driving range. Proper tuning of the two mass flywheel assembly lowers the driveline resonance to frequencies less than engine idle speeds.
While the above two mass flywheel assembly has effective-y eliminated the problem of gear rattle, it has also introduced another problem, namely, resonance mode operation for brief periods during engine start-up/shut-down and in some cases when the engine is lugged at speeds less than normal operating speeds. Torque spikes occurring during these periods are known to be significant enough to damage the flywheel assembly and other components in the driveline system. Such damage is prevented in the prior art flywheel assembly of FIG. 2 by inserting a torque limiting clutch in series between the secondary mass and the torsional attenuating springs of the isolation mechanism. This torque limiting clutch adds cost and complexity to the flywheel assembly. Further, should the torque transmitting capacity of the clutch be set to high or increase for various reasons, the intended safeguard purpose of the clutch is lost. Still further, should the torque transmitting capacity be set to low or decrease during use to values less than are normally encountered, the clutch will continuously slip and soon fail, thereby rendering the vehicle inoperative.